1. Human-Computer Interface Course 5

2. ISPs and Internet connection

3. Domain Naming Service (DNS) DNS provides name to IP address mapping Originally one single central huge table. Today – each domain name provides its own DNS servers. Hierarchical structure: Root DNS servers (serving.com.org.net, etc) Domain serves – serve domain queries.

4. DNS – How it works ? Connect at www.yahoo.comwww.yahoo.com 1. Ask Root Server for the DNS server of.com => a.gtld-servers.net 2. Ask a.gtld-servers.net who is the DNS server for yahoo.com => ns1.yahoo.com 3. Ask ns1.yahoo.com who is www.yahoo.com => is alias for www.yahoo.akadns.netwww.yahoo.comwww.yahoo.akadns.net 4. Ask ns1.yahoo.com who is www.yahoo.akadns.net =>216.109.118.68 www.yahoo.akadns.net

5. The World Wide Web HTML Language – to describe Web pages =>RFC1866 and RFC1942 HTTP protocol – to transmit web pages The Uniform Resource Locator – to name Web pages Hypertext – a way of describing documents and data that reference other documents/data.

6. TCP/IP Peer to peer communication 193.231.20.34 Port: 1563 63.78.171.45 Port: 80 HTTP IPAddress + Port: Identify communicating applications on the source and destination machines

7. TCP/IP – peer to peer IP Address – identifies the computer Port a communication channel descriptor 0-65535 possible ports on one machine 0-1024 – reserved well known ports (ex:80 – Web Servers) Servers listen on well known ports

8. Client/Server –architectures Generally involves two entities Server – application designed to service specialized requests. Clients – applications using the services offered by the server. They are calling the server and ask him specific tasks. Example: Calling telephone information to find someone’s phone number

9. URLs and URIs URL – described in RFC1738 and RFC1808.RFC1738RFC1808 URI – Uniform Resource Identifiers – strings that specify how to access network resources. Ex: URL: http://www.cs.ubbcluj.ro http://www.cs.ubbcluj.ro/math/index.html Download RFCs: http://www.rfc-editor.org/

10. TCP/IP Client Servers Server – applications listening/waiting for client requests on a well known port. Clients – connect to the server and issue requests on the communication channel. Requests are processed by the server and responses are sent back to clients.

11. URL – Locating resources http://www.FreeSoft.org/Connected/index.html http – we are using the HTTP protocol to access the resource. www.Freesoft.orgwww.Freesoft.org - the DNS name of the web server that has our resource. /Connected/index.html – the name of the directory where the server stores our document.

12. HTTP Protocol Allows exchange of HTML and Web data. Works on TCP port 80 and is human readable. Ex: Connect to www.cs.ubbcluj.ro GET / HTTP/1.0 > > < HTTP/1.0 200 OK < Date: Wed, 18 Sep 1996 20:18:59 GMT < Server: Apache/1.0.0 < Content-type: text/html < Content-length: 1579 < Last-modified: Mon, 22 Jul 1996 22:23:34 GMT < < HTML document

13. The File Transfer Protocol-FTP Provides ability to transfer files between machines heterogeneous env. and Op. Syst Works on the port 21 using the TCP protocol. Ex: ftp ftp.ubbcluj.roftp.ubbcluj.ro User: anonymous Password: dadi@cs.ubbcluj.rodadi@cs.ubbcluj.ro ftp> get README

14. Internet Mail Mail protocols: SMTP, IMAP, POP3 SMTP – Simple Mail Transfer Protocol IMAP, POP3 – mailbox access protocols. SMTP – works on TCP – port 25. Each domain name has >=1 mail exchange servers Human readable text protocol Best effort mail transfer protocol.

15. Internet Security Internet connectivity and access to each computer – opportunity for hackers to access distant machines or intercept communications. Protection methods: Firewalls, proxy servers. Encrypted communications Digital signatures

16. Internet Security – Firewalls Firewall – piece of software or hardware implemented component that blocks IP packets for non-authorized users/machines. Proxy Server – software component that intermediates connections between web browsers and external web servers.

17. Encryption, signatures SSL – Secure Sockets Layer – protocol implementing encrypted communication between two hosts with source and destination authentication. Digital signatures allow to identify if a piece of data has been tampered during its transport.

18. Firewalls Internet Computers